Telephone repeater control circuit



Nov. 26, 1940.

LPF

H. W. AUGUSTADT TELEPHONE REPEATER CONTROL CIRCUIT Filed July 11,1959

FIG.

//v VENTOR H W AUGUS TADT A T TORNE V Patented Nov. 26, 1940 W UNITED vs'rares PATENT OFFiCE 2,223,200 TELEPHONE REPEATER. CONTROL omcUrr Herbert W. Augustadt, Valley Stream, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 11, 1939, Serial No. 283,729

- 9 Claims.

is frequently the case in conference circuits, it

is highly desirable that when no normal signaling is taking place the system shall be as quiescent as possible, being-substantially immune from room noise and other background disturbances. Even during a signaling period, such as conversation, it is desirable that in the pauses and intervals of the usual speech the circuits shall be brought to a highly quiescentcondition.

In my invention, as hereindescribed, there is presentin a signal path a vacuum tube amplifier which is so'associated with another circuit, such as another vacuum tube amplifier, that the first amplifier is part of an oscillatory circuit'os'cillating at a frequency outside the signaling range, suchas an ultra-audio frequency. Under these conditions it is found that the vacuum tube amplifier in the signal path is unable to amplify and transmit the signalQ In my circuit, however, when a desired signal is initiated a portion of it isused to develop and supply a voltage which may be applied to the oscillating circuit at'some point and in such manner that oscillations are no longer maintained, whereupon the amplifier in the signal path is enabled, permitting the transmission of signals; As soon as the signal ceases, the oscillatory condition is'reestablished and the amplifier in the signal path is disabled.

Furthermore, when the circuit at one terminal is arranged for two-way conversation, having a transmit side and a receive side, it is important to prevent singing around the circuit of the terminal, which I accomplish by permitting that side over which signal is passing to substantially disable the other side for the time that the signal continues to arrive.

The invention will be better understood by reference to the following specification and the accompanying drawing; in which:

Fig. 1 is a highly simplified schematic drawing of a circuit embodying my'lnvention;

Fig. 2 is a more detailedv drawing of the circuit at one terminal; and I Fig. 3 is a modified form which a portion of the terminal circuit may take. Referring more specifically'to' Fig, 2, there is shown a microphone M which feeds into an amplifier, A1, this amplifier being preferably of a variable mu type. The output R1 of this amplifier feeds into other amplifiers A, if desired, and the signal is then impressed on amplifier A2, the output normally going through transformer T1 to line L.

In the specific form shown in Fig. 2 a second tube 02 is provided which is 50- associated with the tube A2 that the two form an oscillating syst-em, the frequency of which is above the signaling range for which the circuit is to be used.

- While a Wide variety of'vacuum tubes 02 maybe used for this purpose, the one illustrated herein is the so-called 6L7 tube and the frequency of oscillations may be determined by any of the circuit constants, suchfor example, as the anti-* resonant circuit 5. A suitable oscillatory condition for the combination of tubes and circuits is obtained by the application of proper voltages and proper resistances in the various parts of the circuit with proper degree of coupling between the two circuits as indicated at l and 8. When an implifier such as A1 is set into an oscillatory condition in such a manner as described, it is unable to act as an amplifier for a signal impressed on its input circuit. If, however, the oscillatory condition is destroyed for a time, then the amplifier will actin the normal manner to amplify and transmit the signal.

When a signal is initiated at the point M it arrives at the input of the tubeAz but since this) tube is in an oscillatory condition, the signal cannot get through. To enable the circuit, a portion of the output of the tube A1 is diverted through an amplifier 9 and a detector circuit Ill which provides a direct current voltage across the resistance element H and this in turn is applied to the grid 93 of the SL7 tube 02. By suitable adjustment of the amplification by tube 9 and the other constants of the circuit, the negative bias thus placed on the grid is made sufficient to stop the oscillatory condition, whereupon amplifier A2 becomes operative to transmit scribed is one which I find particularly satisfactory, it is to be understood that this is only one of a number of forms which my invention may take. For example, instead of using the tube 02, the amplifier A2 could have been made oscillatory in itself by any suitable feedback arrangement from the input to the output. This feedback circuit could then be opened at any desired point by a relay operated from the amplifier detector circuit Ill. As another alternative one could insert in such afeedback circuit an element, the resistance of which varies rapidly with impressed voltage- For example, a copper-oxide rectifier unit could be inserted in the feedback circuit and the bias from the rectifier circuit l would then be so impressed on the copper-oxide rectifier as to change its resistance and remove the vacuum tube A2 from the oscillatory condition. All such variations are obviousv as are other alternatives which might be suggested, all coming within the purview of my invention.

Such a circuit as described might constitute the transmitend of a four-wire conference circuit. On the other hand, it can readily be adapted as the transmit .side of a two-way two-wire communication channel, as is shown in further detail in Fig. 2. In the latterevent the output from the amplifier A2 would be impressed on-the line L through the hybrid coil H with the balancing network N in a manner now well understood in the art. The receive side of this two-way terminal starting with the connection from the hybrid 'coil is identical to that described in connection with-the transmit side except that it terminates in more suitable responsive devices, such as a loud-speaker LS. Since the circuit arrange ment for thisside isidentical with the transmit side, it will not be necessary to describe it in de-v in the amplifiers" on the two sides is quite high.

and there are slight unbalances in the. circuit, then there is danger of setting up a singing condition within the terminal circuits. To prevent such acondition I provide means whereby the side of the circuitwhich is carrying signal at the moment disables the other side by so reducing its gain that any speech arriving thereat cannot enable its own amplifiers. Thus, consider speech to be initiated at the microphone M. Then there is transmittedfrom the detector circuit l 0 a potential which is impressed on the grid g3 of the tube 02 and the grid 93 of the amplifier As, this being a negative potential of such value as to very greatly reduce the gain of the variable 'mu tube As. It is to be understood that the timing of the operations is such that the amplifier As is disabled before amplifier A is enabled. So long as signal continues at M and a corresponding potential is maintained on grid 3 of tube As, the receive side of the circuit is disabled.

Upon the cessation of signal at M the gain of the tube As is restored to normal and ifsignal then comes in over the line it in turn reduces the gain of the tube A1 in an identical manner, and enables the amplifier A4.

The whole of the circuit as described in Fig. 2

While in Fig. 2 the plate voltage supply for the various tubes is indicated as individual batteries, it is to be understood that in practice one would usually prefer a single B voltage supply which might take on any form, such as a power pack unit. Also, it is evident that for proper operation of the circuits the correct biasing for the difierent tubes is important." Obviously, any suitable method for obtaining such biasing may be used although in the figure I have in general shown self-biasing, as indicated by resistances with by-pass condensers.

In the description above and in the drawing the so-called 6L7 tube has been used in a number of places. I find this a particularly convenient tube because of the ease with which the gain of the tube can be adjusted by voltage bias on thegrid g It is to be understood, however, that my invention is not limited to this form of variable mu tube but that any tube or combination of tubes equivalent thereto may be 'used.

At the terminal the pick-up and the receiver are shown as separate instruments. 'Itwould be desirable in some cases to replace these by a single device serving both as a pick-up and as a receiver. Such a modification is shown in Fig. 3 in which R serves the double function andis associated with the transmit and the receive side of the ter- 1. In a signaling circuit comprising a signal amplifier, means for normally blocking the amplifier as a signal amplifier, said means comprising a circuit includingsaid amplifier normally oscillating at a frequency outside the signal frequency range, and means for rendering theoscillating circuit noneoscillatory to render said amplifier operative to amplif signals, said last means becoming efiec'tive'on receipt of a desired signal wave.

2. In a telephone circuit comprising a line signal amplifier, means for normally blocking the amplifier, said means comprising a second amplifier forming with the first an oscillating circuit normally oscillating at a frequency above the signal frequency range, and means-for rendering the said second amplifier. inoperative to stop the oscillations in said circuit, said last means becoming effective on receipt of adesired signal wave.

3. The combination of claim 2 characterized in that said second amplifier comprises an amplifyin vacuum tube having a control electrode, and the received signal wave applies a negative biasingvoltage to said control electrode to disquency range and thus preventing said amplifier from transmitting-frequencies within said range, and means responsiveto signals initiated in said path to apply a blocking bias :to said vacuum tube to stop oscillation of said oscillating circuit whereby the amplifier is rendered operative to transmit said signalsaq 5. The combination of claim 4, characterized by the fact that said: vacuum tube has a gridcathode circuit including a series resistance and the signal responsive means comprises an amplifier rectifier unit having its' input connected across said path in front ofsaid' amplifier and its output connected across said resistance.

6. In a two-way communication-system, an

amplifier associatedwith the transmit side of one terminal and an amplifier associated with the receive side of that terminal, a vacuum tube associated with each of the said amplifiers and so ad,- justed that each circuit, comprising an amplifier and a vacuum tube, oscillates at a frequency above signal frequency range thus blocking the amplifier for signal frequency and rendering the circuit quiescent, and means responsive to the application of signals of given amplitude to the input of the amplifier in either side of said terminal to render the amplifiervacuum tube combination on that side of the terminal non-oscillatory so that the amplifier of the combination is rendered operative to amplify the applied signals.

7. The combination of claim 6, characterized by the fact that when signal of sufficient amplitude arrives at one side the signal responsive unlblocking means on that side renders the other side substantially inoperative for signal transmission.

8. The combination of claim 6, characterized by the fact that there is a preamplifier for the transmit and a preamplifier for the receive side,

the fact that there are a plurality of such terminals linked together for intercommunlcation HERBERT W. AUGUSTADT. 

